Because attacking people for beliefs you don't like doesn't exactly make things better.
A lot of the people that seem to be trying to attack those they see as facist would be just as bad in the power as those they oppose. Except you know, with different targets.
One rule of thumb would to be to recognize that, within all groups, there are extremists (and they often have the loudest voices). It is wise not to denounce an entire group based on the extremists. It is wise not to flippantly dismiss concerns raised based on the behavior of the extremists.
The problem I see with a heck of a lot of online conversations is that it ridiculously fails this rule. Often times there are some issues on both sides of the coin worth discussing, but it's hard to discuss things rationally when the loudest voices on both sides are acting like spoiled toddlers or often worse. Often times, the extremists dominate the conversation and the resulting discussion is toxic.
Violence is never the solution, but isn't it possible to be anti-fascist (or against any other political movement that more relies on emotionalism instead of rationalism) without being violent? I would think so.
Of course it's possible to be anti facist without being violent. It's possible to non violently get changes in the world, at least if you're in a remotely democratic society like the one you're in at the moment. People like Martin Luther King and Mahatma Gandhi proved it.
Responding to political concerns with violence just begets more violence. It just entrenches people's beliefs even further, inspires revenge from the 'other side' (or at least, their own axe crazy extremists) and eventually causes everything to erupt even further (through a civil war).
Honestly, I'd say this part in the most recent series of Doctor Who sums up some of the issues with people trying to solve all their problems with violence:
Martin Luther King changed the world because America decided that it would rather deal with him, then the far more violent parts of the black liberation movement.
If those more violent parts didn't put the fear of God into the government, he could have been safely ignored.
The standard I often judge groups by is whether it's possible to be a decent person and remain a member of the group; rather than looking at the assholes, I look at how the group treats members who stand up to the assholes. Unfortunately, a lot of groups have been failing this spectacularly, and the older a group the worse it often seems to get.
Because attacking people for beliefs you don't like doesn't exactly make things better.
A lot of the people that seem to be trying to attack those they see as facist would be just as bad in the power as those they oppose. Except you know, with different targets.